Nest Tstat Problems

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mkgrady

Senior Member
Location
Massachusetts
I'm just finishing a home renovation where the owner wants Nest tstats to control heat and AC. The heat is forced hot water baseboard using taco zone control panels. There are 10 tstats total. One is AC only and working with no problems. Six are heat only and working with no problems and three control both heat and ac. These three are having problems. In these three I have tried using the common wire from the heat system and from the ac system.

In either case the heat for these zones comes on briefly, intermittently and sporadically even though it is not calling for heat. The frequency of these malfunctions depends on how many of the heat/cool tstats I connect. If I connect just one there are no problems. As soon as I connect more than one the three heat zones come on. My only thought is to add three more nests to keep the system wiring totally seperate but the owner will not be happy with that solution.
 
You could be overloading the control transformer. Is that a TACO controller
You need to keep the common wires separate from the two systems . Remove the jumpers I think from the R and the RH.
Wow that is an expensive setup.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
I'm just finishing a home renovation where the owner wants Nest tstats to control heat and AC. The heat is forced hot water baseboard using taco zone control panels. There are 10 tstats total. One is AC only and working with no problems. Six are heat only and working with no problems and three control both heat and ac. These three are having problems. In these three I have tried using the common wire from the heat system and from the ac system.

In either case the heat for these zones comes on briefly, intermittently and sporadically even though it is not calling for heat. The frequency of these malfunctions depends on how many of the heat/cool tstats I connect. If I connect just one there are no problems. As soon as I connect more than one the three heat zones come on. My only thought is to add three more nests to keep the system wiring totally seperate but the owner will not be happy with that solution.

Don't tie the commons together, or the r leads. For some reason several years ago some one decided that the secondary of the control xfmr was a SDS and the common should be grounded. that's all well and good, until the r leads get tied together. Then they try to backfeed if you shut one down with the breaker and ask them all to run. Then the magic smoke gets out.......
 

mkgrady

Senior Member
Location
Massachusetts
You could be overloading the control transformer. Is that a TACO controller
You need to keep the common wires separate from the two systems . Remove the jumpers I think from the R and the RH.
Wow that is an expensive setup.
The zone control panel is taco. Actually two that connect together to handle up to ten zones. The two system commons are seperate in that only one is connected at each nest. Where a nest only controls heat the heat common is used. Where it controls both tech support had me remove the heat system common and use the ac common. This worked until I did the same at the second and third nest.
 

mkgrady

Senior Member
Location
Massachusetts
The nests you are having issues with are the ones running the air conditioning? If so is there only one ac system that is zoned? If a yes to that what kinda zone panel is on ac system.
Yes I am having trouble with the three tstats that have heat and ac. The heat only have no problems. Of the three ac tstats one is a single system and one has two zones. Don't know the brand but it does have a see through cover if that means anything

I just looked on the web. It looks like is probably EWC NCM300
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I don't know about Nest, I thought they were grossly over priced so I didn't use them. But I tried to use a web enabled programmable stat from Honeywell on a small project and it too had problems working on a Heat / Cool setup. Turned out that it NEEDS the blue wire connected back to the heater. Not sure why, but it would NOT work right without that, but they DIDN'T run a 4 conductor cable to the t-stat, and I couldn't run a new cable. The unit was roof-top at a condo and the HOA would not allow me to do any work on the roof, only their approved contractors were allowed up there. That guy wanted me to pay him $500 to pull that 12 ft. t-stat cable! So I just told the owner they couldn't have a web enabled t-stat.

Since I couldn't return the Honeywell, I installed it at my own house. I had the extra conductor in my bundle, but it was not used on the old t-stat and was never connected at the heater end, nor was there a terminal on the heater board for it. But after studying the schematics, I figured out that it was only a chassis ground. I ran a separate wire to test that theory and sure enough, that was it. So I just connected the spare (blue) conductor in my t-stat cable to a chassis screw and bingo, worked fine.

Before I figured that out, I had assumed the Honeywell was not going to work on that original project, so I looked at all of the other web-enabled devices like that on the market, looking for one that did NOT need that blue wire connected. No dice. The point is, that was when I looked into the Nest and despite it costing 3x what the Honeywell version did, it too was going to have the same problem; it needs a ground reference back to the heater.
 

mkgrady

Senior Member
Location
Massachusetts
I don't know about Nest, I thought they were grossly over priced so I didn't use them. But I tried to use a web enabled programmable stat from Honeywell on a small project and it too had problems working on a Heat / Cool setup. Turned out that it NEEDS the blue wire connected back to the heater. Not sure why, but it would NOT work right without that, but they DIDN'T run a 4 conductor cable to the t-stat, and I couldn't run a new cable. The unit was roof-top at a condo and the HOA would not allow me to do any work on the roof, only their approved contractors were allowed up there. That guy wanted me to pay him $500 to pull that 12 ft. t-stat cable! So I just told the owner they couldn't have a web enabled t-stat.

Since I couldn't return the Honeywell, I installed it at my own house. I had the extra conductor in my bundle, but it was not used on the old t-stat and was never connected at the heater end, nor was there a terminal on the heater board for it. But after studying the schematics, I figured out that it was only a chassis ground. I ran a separate wire to test that theory and sure enough, that was it. So I just connected the spare (blue) conductor in my t-stat cable to a chassis screw and bingo, worked fine.

Before I figured that out, I had assumed the Honeywell was not going to work on that original project, so I looked at all of the other web-enabled devices like that on the market, looking for one that did NOT need that blue wire connected. No dice. The point is, that was when I looked into the Nest and despite it costing 3x what the Honeywell version did, it too was going to have the same problem; it needs a ground reference back to the heater.
I have the choice of connecting the common from the heat system or the ac system. In fact I am unable to charge the nest battery unless one of the commons is connected. The problem is the nest that control heat and ac malfunction. The heat comes on spoadically in those zones no matter what the nest is calling for
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
I have the choice of connecting the common from the heat system or the ac system. In fact I am unable to charge the nest battery unless one of the commons is connected. The problem is the nest that control heat and ac malfunction. The heat comes on spoadically in those zones no matter what the nest is calling for

I am confused. If you are using a EWC NCM300 type controller wouldn't the AC and the heater be wired to the controller and then the t-stat also wired to the controller.
 

mkgrady

Senior Member
Location
Massachusetts
I am confused. If you are using a EWC NCM300 type controller wouldn't the AC and the heater be wired to the controller and then the t-stat also wired to the controller.
The heat is not wired to the zone air damper controller. The heat is a separate forced hot water system. I'm trying to get the nests to control ac and heat from separate systems
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
The heat is not wired to the zone air damper controller. The heat is a separate forced hot water system. I'm trying to get the nests to control ac and heat from separate systems
I'm not sure I'm fully following this thread, but my understanding is:

1) The Nest has only one place to land the common conductor, for use with an HVAC system with combined A/C and Heat but a single control transformer.

2) You have completely separate A/C and Heat systems, each with its own control transformer.

In that case, I don't think you can mix the conductors from the two different control transformers. If one Nest needs to control both systems, you can wire one system directly to the Nest, and then use a relay powered by that system in conjunction with the Nest to control the other system.

Cheers, Wayne
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
I'm not sure I'm fully following this thread, but my understanding is:

1) The Nest has only one place to land the common conductor, for use with an HVAC system with combined A/C and Heat but a single control transformer.

2) You have completely separate A/C and Heat systems, each with its own control transformer.

In that case, I don't think you can mix the conductors from the two different control transformers. If one Nest needs to control both systems, you can wire one system directly to the Nest, and then use a relay powered by that system in conjunction with the Nest to control the other system.

Cheers, Wayne
I agree. A relay for each heater is what I would do.
 

mkgrady

Senior Member
Location
Massachusetts
I'm not sure I'm fully following this thread, but my understanding is:

1) The Nest has only one place to land the common conductor, for use with an HVAC system with combined A/C and Heat but a single control transformer.

2) You have completely separate A/C and Heat systems, each with its own control transformer.

In that case, I don't think you can mix the conductors from the two different control transformers. If one Nest needs to control both systems, you can wire one system directly to the Nest, and then use a relay powered by that system in conjunction with the Nest to control the other system.

Cheers, Wayne

I believe the nest is made to be able to control two seperate systems with two seperate control systems. In fact it works just fine until I connect the second nest that has heat and ac. At that point the nest that have ac connected turns the heat on.

I called nest today and was put on hold for over half an hour then I gave up. While on hold they encourage you to go to the online support. That was worthless because it assumes you have no technical knowledge. All the subjects are simple and my problem is not addressed at all.

About your suggestion, I think the zone control panel is just a bunch of relays so I'm confused as to what you think will work.
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
I believe the nest is made to be able to control two seperate systems with two seperate control systems. In fact it works just fine until I connect the second nest that has heat and ac. At that point the nest that have ac connected turns the heat on.

I called nest today and was put on hold for over half an hour then I gave up. While on hold they encourage you to go to the online support. That was worthless because it assumes you have no technical knowledge. All the subjects are simple and my problem is not addressed at all.

About your suggestion, I think the zone control panel is just a bunch of relays so I'm confused as to what you think will work.

You said the heat did not go through the zone control. If it doesn't then you could wire the ac part to the zone control and use a relay energized from W and C on the t-stat to control heat.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
three control both heat and ac. These three are having problems. In these three I have tried using the common wire from the heat system and from the ac system.
FWIW, this link:

https://nest.com/support/article/Power-Specifications-for-the-Nest-Learning-Thermostat

says that if both Rc, Rh, and C are connected, the Nest will draw its power from Rc and C. So for the three Nests connected to both the heating and cooling systems, you should connect the common from the cooling system.

Maybe if all three of the heating/cooling Nests have their common taken from the cooling system, the weird problem will go away?

Cheers,
Wayne
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
If you have one transformer for all three heaters also verify all three Rh wires are on the same leg of the transformer and that they are not on the bonded leg. Make sure your Rc is not on the bonded leg for the AC. Also look for voltage drop when all three are connected. If all the wiring is good I would add a relay in each heater to control the heaters.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
FWIW, this link:

https://nest.com/support/article/Power-Specifications-for-the-Nest-Learning-Thermostat

says that if both Rc, Rh, and C are connected, the Nest will draw its power from Rc and C. So for the three Nests connected to both the heating and cooling systems, you should connect the common from the cooling system.

Maybe if all three of the heating/cooling Nests have their common taken from the cooling system, the weird problem will go away?

Cheers,
Wayne

If you have one transformer for all three heaters also verify all three Rh wires are on the same leg of the transformer and that they are not on the bonded leg. Make sure your Rc is not on the bonded leg for the AC. Also look for voltage drop when all three are connected. If all the wiring is good I would add a relay in each heater to control the heaters.

you need to keep the system transformers separate otherwise you will have continued problems.

I would place 1 relay for each of those 3 heat zones that turns on the TACO valve and pump powered by the output of the Nest. I see no other way as the output from the nest does not allow for two control systems. This should work as long as the AC is not powered off.

You can do this the other way and use relays for the AC it might get more complex though.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
the output from the nest does not allow for two control systems.
That is what I originally thought, but then I reconsidered: If a thermostat has separate terminals for Rc and Rh, and has its own power (say batteries), then why can't it control two separate systems with separate control transformers? When it calls for cooling it will connect Rc to the cooling wire (Y?), and when it calls for heating, it will connect Rh to the heating wire (W?).

In the case of the Nest, it wants power from the control system, so it has a C common terminal, but the link I provided tells you which common to use when you have two separate control transformers: use the common associated with Rc. So now aren't we back to the above situation?

Cheers,
Wayne
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
That is what I originally thought, but then I reconsidered: If a thermostat has separate terminals for Rc and Rh, and has its own power (say batteries), then why can't it control two separate systems with separate control transformers? When it calls for cooling it will connect Rc to the cooling wire (Y?), and when it calls for heating, it will connect Rh to the heating wire (W?).

In the case of the Nest, it wants power from the control system, so it has a C common terminal, but the link I provided tells you which common to use when you have two separate control transformers: use the common associated with Rc. So now aren't we back to the above situation?

Cheers,
Wayne

I would agree , But I don't have a real wiring diagram the there are no jumpers to remove. so all the relay outputs are connected to the same source.
It appears that way.
 
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